It is common practice for an airplane to have links forming structural parts. This applies for example in jet engine masts, in landing gear, in floor structures, in wing box ribs, etc. These links are subjected to traction or compression forces and they are sometimes associated with one another in other to form a trellis type structure. Nevertheless, in a structure of that type, as in other arrangements, it often happens that each link is subjected mainly to a single kind of stress, namely pure traction or pure compression.
The dimensioning of a structural zone of an airplane, e.g. a trellis of links, must satisfy specific specifications associated with the mechanical strength of each link relative to predetermined stress, with its stiffness, and with its size. In general, it is desirable to minimize the weight and the cost of such links, while complying with the specifications. More particularly, it is desirable under such circumstances to minimize the weight of a link that is subjected to mechanical stress of predetermined magnitude.
In this context, it is known to use links made of metal or else links made of composite material. Each of those technologies has its advantages and its drawbacks.
Consideration is given initially to a link that is stressed purely in traction. Its dimensioning is associated directly with the level of its elongation. If the link is made of metal, it must be ensured that it is not subjected to plastic deformation prior to reaching limit loads, and that it does not break prior to reaching extreme loads. If the link is made of composite material, it must be ensured that it satisfies the damage-tolerance criterion that is determined by the acceptable level of elongation under extreme loads.
By way of example, consideration is given to a traction force of 20 (metric) tonnes (t) or 200,000 newtons (N). The behavior of a link made of aluminum of reference 2024T42 is compared with the behavior of a pre-impregnated composite material link having carbon fibers with an intermediate Young's modulus and oriented in the 50/20/20/10 mode, i.e. 50% at 0° relative to the reference direction, 20% at −45°, 20% at 45°, and 10% at 90°. In operation, the metal link is subjected to greater elongation than the composite link. Thus, the section of the metal link may be selected as being smaller than the section of the composite link. Nevertheless, the metal link is of greater density (relative density 2.7) than the composite link (relative density 1.6). For example, if a composite material link and a metal link present substantially identical performance in traction, the composite material link is only about 22% lighter than the metal link in spite of having density that is 41% smaller. It is the damage tolerance effect that makes the composite material link less effective, since it operates with a rather low level of elongation. No consideration is given here to the fatigue criterion, but it is penalizing for the metal link.
Consideration is now given to a link that is subjected to pure compression. Depending on its length, its dimensioning may be associated either with its section (as applies to a link that is short, with its section being dimensioned as a function of elongation), or else with its second moment of area or “inertia” (as applies to a link that is long, with its inertia being dimensioned as a function of buckling). As well as verifying as above that there is no plastic deformation or breaking and also verifying that the damage tolerance criterion is complied with, it is necessary to verify compliance with the buckling criterion for both types of link.
By way of example, consideration is given to short links, i.e. links having a length of 700 millimeters (mm), that are subjected to a compression force of 20 t and that are made respectively out of the same materials as described above. The metal link is subjected to a greater amount of elongation than the composite link. Thus, the section of the metal link may be selected to be 44% smaller than the section of the composite link. However, the metal link is of greater density than the composite link. In the end, for a link that is not dimensioned for buckling (a short link), and with equivalent levels of performance in compression, it is possible for a metal link to be made 5% lighter than a composite link. This difference, which is small in spite of the great difference in densities between the materials, is the result of the damage-tolerance effect that makes the composite link less effective because it is subjected to a relatively small level of elongation.
Consideration is now given to longer links, e.g. links that are 1.8 meters (m) long, and that are subjected to a compression stress of 20 t, and made respectively out of the same materials as above. The Young's modulus of the composite link (89,430 megapascals (MPa)) is slightly greater than that of the metal link (76,500 MPa). With unchanging shape, the bending stiffness (Young's modulus and section modulus) of the composite link is thus slightly more favorable in terms of the buckling criterion than is the bending stiffness of the metal link. Furthermore, the metal link is of greater density than the composite link. This leads to a link that is dimensioned in terms of buckling possibly being 40% lighter when made of composite material than when made of metal. This results mainly from the effect of the difference in density. Under such circumstances, the composite link can be said to be fully effective since it is does not need to work at a high level of elongation.
Thus, when a link is dimensioned in terms of buckling under compression stress, it is more effective when made of composite material than of metal. The weight saving achieved is close to the density saving that corresponds to the materials. In contrast, when the link is not dimensioned in terms of buckling (either for traction stress, or for compression stress when the link is short), composite material is not so effective because of the damage-tolerance criterion, it being understood that the working level of elongation for a composite link is much less than for a metal link.
Both of those two solutions are therefore imperfect.